Lubricating oil compositions for use in crankcase engine oils comprise a major amount of base stock oil and minor amounts of additives that improve the performance and increase the useful life of the lubricant. Crankcase lubricating oil compositions conventionally contain polymeric components that are used to improve the viscometric performance of the engine oil, i.e., to provide multigrade oils such as SAE 5W-30, 10W-30 and 10W-40. These viscosity performance enhancers, commonly referred to as viscosity index (VI) improvers, include olefin copolymers, polymethacrylates, alkenyl arene/hydrogenated diene block and star copolymers and hydrogenated idiene linear and star polymers. From an optimized performance/minimized cost perspective, linear alkenyl arene/hydrogenated diene block copolymer VI improvers are favored by many lubricating oil blenders.
VI improvers are commonly provided to lubricating oil blenders as a concentrate in which the VI improver polymer is diluted in oil to allow, inter alia, for dissolution of the VI improver in the base stock oil. Linear alkenyl arene/hydrogenated diene block copolymer VI improver concentrates usually have lower active polymer concentrations and present greater handleability issues compared to star copolymer or olefin copolymer concentrates. Functionalization of the linear the alkenyl arene/hydrogenated diene block copolymer further exacerbates the handleability issues. A typical linear styrene/hydrogenated diene block copolymer VI improver concentrate may contain as little as 3 mass % active polymer (with the remainder being diluent oil), as higher concentrations of these polymers results in a reduction in the flowability of the concentrates at temperatures at which lubricants are blended. A typical formulated multigrade crankcase lubricating oil may, depending on the thickening efficiency (TE) of the polymer, require as much as 3 mass % of active VI improver polymer. An additive concentrate providing this amount of polymer can introduce as much as 20 mass %, based on the total mass of the finished lubricant, of diluent oil.
As the additive industry is highly competitive from a pricing standpoint, and diluent oil represents one of the largest raw material costs to the additive manufacturers, VI improver concentrates have commonly contained the least expensive oil capable of providing suitable handling characteristics; usually a solvent neutral (SN) 100 or SN150 Group I oil. Using such conventional VI improver concentrates, the finished lubricant formulator has needed to add a quantity of relatively high quality base stock oil (Group II or higher) as a correction fluid to insure the viscometric performance of the formulated lubricant remains within specification.
As lubricating oil performance standards have become more stringent, there has been a continuing need to identify components capable of conveniently and cost effectively improving overall lubricant performance. Therefore, it would be advantageous to be able to provide a linear alkenyl arene/hydrogenated diene block copolymer VI improver concentrate that has an increased active polymer concentration while maintaining acceptable flow properties at temperatures at which lubricants are typically blended.